Method and arrangement of controlling of percussive drilling based on the stress level determined from the measured feed rate

ABSTRACT

The rock drill apparatus ( 1 ) comprises a rock drill machine ( 6 ) provided with a percussion device ( 4 ), a feed device ( 9 ) and a tool ( 7 ), the tool ( 7 ) end comprising a bit ( 8 ) for breaking rock. The tool ( 7 ) is arranged to transmit impact energy generated by the percussion device ( 4 ) as a compression stress wave to the bit. The feed device ( 9 ) is arranged to thrust the tool ( 7 ) and the bit ( 8 ) against the rock to be drilled, whereby on drilling at least part of the compression stress wave generated by the percussion device ( 4 ) to the tool ( 7 ) reflects from the rock to be drilled back to the tool ( 7 ) as tensile stress, and impact energy of the percussion device ( 4 ) is adjusted on the basis of the level of tensile stress (σ v ) reflecting from the rock.

FIELD OF THE INVENTION

The invention relates to a method in connection with a rock drillapparatus, which rock drill apparatus comprises a rock drill machineprovided with a percussion device, a feed device and a tool, the toolend comprising a bit for breaking rock, and the tool being arranged totransmit impact energy generated by the percussion device as acompression stress wave to the bit and the feed device being arranged tothrust the tool and the bit against the rock to be drilled, whereby ondrilling at least part of the compression stress wave generated by thepercussion device to the tool reflects from the rock to be drilled backto the tool as tensile stress.

The invention further relates to an arrangement in connection with arock drill apparatus, which rock drill apparatus comprises a rock drillmachine provided with a percussion device, a feed device and a tool, thetool end comprising a bit for breaking rock, and the tool being arrangedto transmit impact energy generated by the percussion device as acompression stress wave to the bit and the feed device being arranged tothrust the tool and the bit against the rock to be drilled, whereby ondrilling at least part of the compression stress wave generated by thepercussion device to the tool reflects from the rock to be drilled backto the tool as tensile stress.

BACKGROUND OF THE INVENTION

Rock drill machines are employed for drilling and excavating rock e.g.in underground mines, opencast quarries and on land construction sites.Known rock drilling and excavating methods include cutting, crushing andpercussing methods. Percussion methods are most commonly in use inconnection with hard rock types. In the percussion method the tool ofthe drill machine is both rotated and struck. Rock breaks, however,mainly by the effect of an impact. The main function of the rotation isto make sure that buttons or other working parts of the drill bit or bitat the outer end of the tool always hit a new spot in the rock. The rockdrill machine generally comprises a hydraulically operated percussiondevice, whose percussion piston provides the tool with the necessarycompression stress waves and a rotating motor that is separate from thepercussion device. In the percussion method efficient breaking of rockrequires that the bit be against the rock surface at the moment ofimpact. The impact energy of the percussion device strike produces inthe tool a compression stress wave, which is transmitted from the toolto the bit arranged in the tool end and therefrom further to the rock.Generally, in all drilling conditions part of the compression stresswave reflects back to the tool as tensile stress. If the rock is softand the rock/bit contact is poor the level of tensile stress is high inthe wave reflecting from the rock. If drilling is continued into softrock with excessive impact energy it generally results in worn threadedjoints between the drill rods and/or premature fatigue failures of thedrilling tool.

In general, the method that is currently used for drilling control, aso-called feed-impact-followup-control method, is not able to preventdrilling into soft rock with excessive impact energy. In thefeed-impact-followup-control method the impact pressure is controlled onthe basis of the feed of the drilling machine. The interdependence ofthe impact pressure and the feed pressure in rock drilling is presentedin U.S. Pat. No. 5,778,990, for instance. When soft rock is drilled, thefeed pressure remains in the set value. Only, if the velocity limit setfor the feed of the drilling machine is exceeded, the feed pressuredrops and the pressure of the impact along with it. However, in asituation, for instance, where the feed-impact-followup-control methodis used for drilling from hard to soft rock, the penetration rate of thedrilling rises. In practice, it is impossible to set the velocity limitof the feed to be sufficiently accurate for penetration rate values ofdifferent rock types, in order for the velocity limit of thefeed-impact-followup-control to restrict the feed pressure in a desiredmanner. Because the penetration rate of the drilling thus remains belowthe velocity control limit set for the feed, the feed pressure andconsequently the impact pressure remain at the original level, whichresults in high tensile stress in the tool. Generally speaking, thevelocity limit is constant and it is set so high that it will not detectchange in rock type, but only drilling into a void.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel solution toadjust impact energy of a drilling machine.

The method of the invention is characterized by adjusting impact energyof the percussion device on the basis of the level of tensile stressreflecting from the rock to be drilled to the tool.

The arrangement of the invention is characterized in that impact energyof the percussion device is arranged such that it is adjusted on thebasis of the level of tensile stress reflecting from the rock to bedrilled to the tool.

The basic idea of the invention is that in a rock drill apparatuscomprising a rock drill machine provided with a percussion device, afeed-device and a tool, the tool end comprising a bit for breaking rock,and the tool being arranged to transmit impact energy generated by thepercussion device as a compression stress wave to the bit and the feeddevice being arranged to thrust the tool and the bit against the rock tobe drilled, whereby on drilling at least part of the compression stresswave generated by the percussion device to the tool reflects from therock to be drilled back to the tool as tensile stress, impact energy ofthe percussion device is adjusted on the basis of the level of thetensile stress reflecting from the rock to be drilled to the tool.According to a first embodiment of the invention the level of thetensile stress reflecting from the rock to the tool is determined on thebasis of the interdependence of the drilling penetration rate and thetensile stress level. According to a second embodiment of the inventionthe interdependence of the drilling penetration rate and the tensilestress level is utilized by setting an impact pressure to be used in thepercussion device, setting the highest allowed tensile stress level, towhich the tool of the rock drill machine is subjected, determining thehighest allowed penetration rate of drilling on the basis of the impactpressure used and the highest allowed tensile stress level, determiningthe actual penetration rate of drilling, comparing the actualpenetration rate of drilling with the highest allowed penetration rateand if the actual penetration rate exceeds the highest allowedpenetration rate the operation of the rock drill machine is adjustedsuch that the impact energy of the percussion device reduces to a level,where the actual penetration rate is at most equal to the highestallowed penetration rate of drilling, whereby the tensile stress level,to which the tool of the rock drill machine is subjected, remains belowthe set highest allowed tensile stress level.

The invention has an advantage that it is possible to affect the loadingof the drilling tool directly in a simple manner and thus to affect theservice life of the tool, and that it is possible to adjust the impactenergy accurately to suit various rock types. Implementation of thesolution only requires measurement of the drilling penetration rate, noother measurements are necessarily needed. Controllability of thedrilling improves considerably, because the feed-impact-followup-controlmethod does not react at all if there is no change in the feed pressure.Furthermore, the solution provides information on hardness of the rockat that moment with a given accuracy.

In the following, the present document will also use another parameter,penetration resistance of rock, in addition to rock hardness. Inaccordance with the definition, the penetration resistance of rockdescribes the relation between a drill bit or bit penetration and theforce resisting it, which mainly depends on hardness of the rock andgeometry of the drill bit or bit. Thus, the penetration resistanceconsiders both given characteristics of the drill bit or bit and thehardness of the rock.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail inconnection with the attached drawings, wherein

FIG. 1 is a schematic side view of a rock drill apparatus, to which thesolution of the invention is applied;

FIG. 2 shows schematically tensile stress produced by a rock drillmachine unit strike with different penetration resistances of rock;

FIG. 3 shows schematically penetration of a bit button produced by arock drill machine unit strike or unit impact with different penetrationresistances of rock;

FIG. 4 shows schematically interdependence of impact velocity and impactpressure of a percussion device in a rock drill machine;

FIG. 5 shows schematically interdependence of impact frequency andimpact pressure of a percussion device in a rock drill machine; and

FIG. 6 shows schematically the highest allowed penetration rates of adrilling tool at different tensile stress levels.

FIG. 1 shows a schematic and highly simplified side view of a rock drillapparatus 1, in which the solution of the invention is utilized. Therock drill apparatus 1 of FIG. 1 comprises a boom 2, at the end of whichthere is a feed beam 3 which comprises a rock drill machine 6 includinga percussion device 4 and a rotating device 5. The rotating device 5transmits to a tool 7 continuous rotating force by the effect of which abit 8 connected to the tool 7 changes its position after an impact andwith a subsequent impact strikes a new spot in the rock. Conventionallythe percussion device 4 comprises a percussion piston that moves by theeffect of pressure medium, which percussion piston strikes the rear endof the tool 7 or a shank arranged between the tool 7 and the percussiondevice 4. Naturally, the structure of the percussion device 4 can alsobe of some other type. For instance, it is possible to produce theimpact pulse with means based on electromagnetism. Percussion devicesbased on a property of this kind are also regarded as percussion devicesherein. The rear end of the tool 7 is connected to the rock drillmachine 6 and the outer end or end of the tool 7 comprises a fixed ordetachable bit 8 for breaking rock. During drilling, the bit 8 is thrustwith a feed device 9 against the rock. The feed device 9 is arranged inthe feed beam 3, and the rock drill machine 6 is arranged movably inconnection therewith. Typically the bit 8 is a so-called drill bit withbit buttons 8 a, but other bit structures are also possible. When deepholes are drilled, i.e. in so-called extension rod drilling, drill rods10 a to 10 c, whose number depends on the depth of the hole to bedrilled and which constitute the tool 7, are arranged between the bit 8and the drilling machine 6.

In FIG. 1 the rock drill apparatus 1 is shown considerably smaller thanit is in reality as compared with the structure of the rock drillmachine 6. For the sake of clarity, the rock drill apparatus 1 of FIG. 1only comprises one boom 2, feed beam 3, rock drill machine 6 and feeddevice 9, but it is apparent that the rock drill apparatus is typicallyprovided with a plurality of booms 2 and a feed beam 3 provided with arock drill machine 6 and a feed device 9 is arranged at the end of eachboom 2. Further, it is apparent that generally the rock drill machine 6also comprises a flushing device for preventing the bit 8 from blocking,but for the sake of clarity the flushing device is omitted in FIG. 1.

The impact energy produced by the percussion device 4 is transmitted asa compression stress wave through the drill rods 10 a to 10 c towardsthe bit 8 at the end of the outermost drill rod 10 c. When thecompression stress wave reaches the bit 8, the bit 8 and the bit buttons8 a therein strike the matter to be drilled causing intense compressionstress, by the effect of which fractures are formed in the rock to bedrilled. If the impact energy of the percussion device 4 is excessive ascompared with the rock hardness a problem arises that the tensile stresslevel in the drilling tool becomes unnecessarily high. If drilling iscontinued into soft rock with excessive impact energy it generally leadsto worn threaded joints between the drill rods 10 a to 10 c and/orpremature fatigue failures of the drilling tool.

The solution of the invention for adjusting the impact energy is basedon the fact that it is possible to calculate for each drillmachine/tool/bit combination a stress level caused in the tool 7 by aunit impact with different penetration resistances of rock. The unitimpact is an impact whose velocity v_(i) is 1 m/s. FIG. 2 showsschematically unit tensile stress σ_(v) ^(l) caused by the unit impactwith different penetration resistances K_(l) of rock, the penetrationresistance varying between K_(l)=10˜1000 kN/mm. For one bit type thepenetration resistance of rock in soft rock is K_(l)=10 kN/mm, andcorrespondingly for one bit type the penetration resistance of rock inhard rock is K_(l)=1000 kN/mm. The horizontal axis in FIG. 2 presentsthe penetration resistance of rock K_(l) and the vertical axis presentsthe reflected unit tensile stress σ_(v) ^(l).

An impact at velocity v_(l) causes to the tool a tensile stress level ofσ_(v) =v _(i)σ_(v) ^(l),  (1)where σ_(v) ¹ is the tensile stress corresponding to the unit impactwith a given penetration resistance of rock K_(l) as shown in FIG. 2.Thus, an impact at velocity v_(i)=9.5 m/s into rock, whose penetrationresistance is K_(l)=300 kN/mm, causes to the tool tensile stress ofσ_(v)=9.5*12=114 MPa in accordance with formula (1). Correspondingly,the same impact makes the bit buttons 8 a of the drill bit 8 topenetrate as follows:u _(n) =v _(i) u _(n) ^(l),  (2)where u_(n) ^(l) is the penetration of the bit button 8 a, correspondingto the unit impact, with a given penetration resistance K_(l), as shownschematically in FIG. 3. For instance, an impact at velocity v_(i)=9.5m/s into matter whose penetration resistance is K_(l)=300 kN/mm, causesbutton penetration u_(n)=9.5*0.125=1.19 mm.

Net penetration rate NPR of drilling can be estimated by formulaNPR=αf(u _(n))^(β),  (3)where f is impact frequency, α and β are constants which represent therelation between the penetration of the drill bit buttons and the wholedrill bit. The constants α and β depend on the diameter of the hole tobe drilled and the drill bit geometry, and they can be defined with asufficient accuracy on the basis of the diameter of the outermost buttonin the drill bit, the diameter of the drill bit and the number of theoutermost buttons. Further, it is possible to determine characteristiccurves for each drilling machine, which curves describe how the impactvelocity v_(i) and the impact frequency f depend on the impact pressure.During the drilling, the impact frequency f can be measured e.g. frompressure medium pulsation of the drilling machine. FIG. 4 showsschematically the interdependence of the percussion device impactvelocity v; and impact pressure, on the horizontal axis the impactpressure is given in bars and on the vertical axis the impact velocityof the percussion piston of the percussion device 4 is given in metresper second. FIG. 5, in turn, shows schematically the interdependence ofthe impact frequency f and the impact pressure, on the horizontal axisthe impact pressure is given in bars and on the vertical axis the impactfrequency of the percussion piston of the percussion device 4 is givenin hertz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An adjustment curve required for impact energy adjustment is obtained inthe following manner:

-   -   1. set the highest allowed tensile stress level σ_(v) ^(max).    -   2. determine impact velocity v_(i) and impact frequency f        corresponding to each impact pressure.    -   3. from the impact velocity v_(i) obtained at point 2, search,        by means of formula (1) and the curve of FIG. 2, for the lowest        allowed penetration resistance value K_(l) ^(min), which permits        the tensile stresses to remain below the highest allowed value        σ_(v) ^(max).    -   4. the highest allowed button penetration value u_(n) ^(max)        corresponding to the lowest allowed rock penetration resistance        value K_(l) ^(min) is obtained by formula (2) and by means of        the curve in FIG. 3.    -   5. the highest allowed penetration rate NPR^(max) is obtained        from formula (3), when constants α and β, impact frequency f and        the highest allowed button penetration value u_(n) ^(max) are        known. In this manner, it is possible to determine for the set        tensile stress levels the penetration rate curves describing the        highest allowed penetration rate NPR^(max) as a function of        impact pressure.    -   6. if the highest allowed penetration rate NPR^(max) is exceeded        during the drilling, the highest allowed tensile stress level        σ_(v) ^(max) is also exceeded. Therefore impact pressure should        be reduced so as to reduce the tensile stresses.

If a drilling machine is used, where the stroke length of the percussionpiston of the percussion device 4 can be changed, the impact velocityv_(i) can be reduced, for instance, by adjusting the stroke length,whereby the impact frequency f increases correspondingly. The impactpower then remains constant, but the impact energy reduces to theallowed level. The adjustment curves are then slightly different,because a change in impact frequency f have to be taken into account.

EXAMPLE

FIG. 6 shows schematically, in continuous lines, the highest allowedpenetration rates NPR^(max) in one drilling tool at different tensilestress levels σ_(v). The broken lines are auxiliary lines describing thepenetration resistance K_(l) of the rock to be drilled, which help inperceiving the penetration rates NPR with different penetrationresistances K_(l) of the rock to be drilled and different impactpressures. Initially, the drilling takes place at an operating point A,where the impact pressure is 220 bar and the penetration resistance ofthe rock is about 300 kN/mm. The highest allowed tensile stress levelσ_(v) ^(max) set by the drilling machine operator is 140 MPa. Thedrilling penetration rate at the operating point A is 3.1 m/min, so thepenetration rate is lower than the highest allowed penetration rateNPR^(max)=3.5 m/min corresponding to said impact pressure. As thedrilling proceeds, the rock suddenly becomes softer to the penetrationresistance value K_(l)=200 kN/mm, which refers to the operating point Bof FIG. 6, where the penetration rate is 3.9 m/min, i.e. the penetrationrate is higher than what is allowed for said impact pressure. Theadjustment solution responds to this by dropping the impact pressureuntil the operating point C is attained, where the impact pressure is175 bar and the penetration rate is 3.3 m/min, which is the highestpenetration rate allowed for said impact pressure in said hardness ofthe matter to be drilled.

The solution of the invention permits that it is possible to affect theloading of the drilling tool directly in a simple manner and thus toaffect the service life of the tool. It is possible to adjust the impactenergy accurately to suit various rock types. Implementation of thesolution only requires the measurement of the drilling penetration rate,no other measurements are necessarily needed. The solution improves thecontrollability of the drilling considerably, because thefeed-impact-followup-control method does not react at all if there is nochange in the feed pressure. Furthermore, the solution providesinformation on hardness of the rock to be drilled at that moment with agiven accuracy. Further, if the drilling machine is provided withadjustable stroke length, it is possible to adjust impact frequency andimpact rate, instead of impact pressure, to be suitable for the rockhardness such that the impact energy reduces but the impact powerremains approximately constant.

The penetration rate NPR of the drilling machine is measured on thebasis of the measurement performed by a measuring means 11 arranged inconnection with the drilling machine 6. The measuring means 11 canmeasure directly propagation velocity of the drilling machine 6 on thefeed beam 3, or it can measure the travel of the drilling machine 6 onthe feed beam 3, whereby penetration rate of drilling can be determinedon the basis of the travel made and the time spent. The measurementmessage of the measuring means 11 is transmitted to a control unit 12,which is advantageously a micro-processor- or signal-processor-baseddata processing and control device, which determines a control signal 14to be applied to a pump 13 on the basis of the measurement signalprovided by the measuring means 11 and default values set by theoperator. The default values set by the operator include the impactpressure HP of the percussion device 4 when starting the drilling andthe highest allowed tensile stress level σ_(v) ^(max), during thedrilling. On the basis of these two initial values the control unit 12determines, in the above-described manner, the highest allowedpenetration rate NPR^(max), with which the penetration rate measured bythe measuring means 11 is compared. If the measured penetration rateexceeds the highest allowed penetration rate NPR^(max) the impactpressure of the percussion device 4 is reduced. The pump 13 pumpspressure fluid through a pressure channel 15 in the direction of arrow Ainto the percussion device 4 to produce a stroke of the percussionpiston. During the reverse stroke of the percussion piston the pressurefluid flows through a return channel 16 into a container 17 in thedirection of arrow B. For the sake of clarity, the structure of thepercussion device is only shown schematically in FIG. 1, and forinstance, one or more control valves that are used for controlling thepercussion device in a manner known per se have been omitted in FIG. 1.

The drawings and the relating description are only intended toillustrate the inventive idea. The details of the invention may varywithin the scope of the claims. Hence, instead of being hydraulicallyoperated, the drilling machine can also be a pneumatically orelectrically operated drilling machine.

1. A method in connection with a rock drill apparatus, which rock drillapparatus comprises a rock drill machine provided with a percussiondevice, a feed device and a tool, the tool end comprising a bit forbreaking rock, and the tool being arranged to transmit impact energygenerated by the percussion device as a compression stress wave to thebit and the feed device being arranged to thrust the tool and the bitagainst the rock to be drilled, whereby on drilling at least part of thecompression stress wave generated by the percussion device to the toolreflects from the rock to be drilled back to the tool as tensile stress,the method comprising determining the penetration rate, determining thelevel of tensile stress reflecting from the rock to be drilled to thetool on the basis of interdependence of drilling penetration rate andthe level of the tensile stress and adjusting impact energy of thepercussion device on the basis of the level of tensile stress reflectingfrom the rock to be drilled to the tool.
 2. A method as claimed in claim1, comprising setting an impact pressure to be used in the percussiondevice, setting the highest allowed tensile stress level, to which thetool of the rock drill machine is subjected, determining the highestallowed penetration rate of drilling on the basis of the impact pressureused and the highest allowed tensile stress level, determining theactual penetration rate of drilling, comparing the actual penetrationrate of drilling with the highest allowed penetration rate, and if theactual penetration rate exceeds the highest allowed penetration rateadjusting the operation of the rock drill machine such that the impactenergy of the percussion device reduces to a level, where the actualpenetration rate is at most equal to the highest allowed penetrationrate of drilling, whereby the tensile stress level, to which the tool ofthe rock drill machine is subjected, remains below the set highestallowed tensile stress level.
 3. A method as claimed in claim 2, whereinthe actual penetration rate of the drilling is determined by measuringthe proceeding rate of the rock drill machine of the feed beam.
 4. Amethod as claimed in claim 1, wherein the actual penetration rate of thedrilling is determined by measuring the proceeding rate of the rockdrill machine on a feed beam.
 5. A method as claimed in claim 1, whereinthe impact energy of the percussion device is adjusted by changing theimpact pressure of the percussion device.
 6. A method as claimed inclaim 1, wherein the stroke length of a percussion piston of thepercussion device is adjustable and the impact energy of the percussiondevice is adjusted by changing the stroke length of the percussionpiston of the percussion device.
 7. An arrangement in connection with arock drill apparatus, which rock drill apparatus comprises a rock drillmachine provided with a percussion device, a feed device and a tool, thetool end comprising a bit for breaking rock, and the tool being arrangedto transmit impact energy generated by the percussion device as acompression stress wave to the bit and the feed device being arranged tothrust the tool and the bit against the rock to be drilled, whereby ondrilling at least part of the compression stress wave generated by thepercussion device to the tool reflects from the rock to be drilled backto the tool as tensile stress, the arrangement comprising a measuringmeans for determining drilling penetration rate and a control unit thatis arranged to determine the level of tensile stress reflecting from therock to be drilled to the tool on the basis of interdependence of thepenetration rate of the drilling and the level of tensile stress and inwhich arrangement the impact energy of the percussion device is arrangedto be adjusted on the basis of the level of tensile stress reflectingfrom the rock to be drilled to the tool.
 8. An arrangement as claimed inclaim 7, wherein the control unit comprises means for setting an impactpressure to be used in the percussion device, setting the highestallowed tensile stress level, to which the tool of the rock drillmachine is subjected, determining the highest allowed penetration rateof drilling on the basis of the impact pressure used and the highestallowed tensile stress level, determining the actual penetration rate ofdrilling, comparing the actual penetration rate of drilling with thehighest allowed penetration rate, and if the actual penetration rateexceeds the highest allowed penetration rate adjusting the operation ofthe rock drill machine such that the impact energy of the percussiondevice reduces to a level, where the actual penetration rate is at mostequal to the highest allowed penetration rate of drilling, whereby thetensile stress level, to which the tool of the rock drill machine issubjected, remains below the set highest allowed tensile stress level.9. An arrangement as claimed in claim 8, comprising a measuring means,which is arranged to determine the actual penetration rate of thedrilling by measuring the proceeding rate of the rock drill machine onthe feed beam.
 10. An arrangement as claimed in claim 7, wherein theactual penetration rate of the drilling is determined by measuring theproceeding rate of the rock drill machine on the a feed beam.
 11. Anarrangement as claimed in claim 7, wherein the impact energy of thepercussion device is adjusted by changing the impact pressure of thepercussion device.
 12. An arrangement as claimed in claim 7, wherein thestroke length of a percussion piston of the percussion device isadjustable and the impact energy of the percussion device is adjusted bychanging the stroke length of the percussion piston of the percussiondevice.